1. Field of the Invention
This invention relates to a method for making propylene utilizing hydrocarbons that have four carbon atoms per molecule (C4's). More particularly this invention relates to a process for forming propylene from an isobutylene containing feedstock.
2. Description of the Prior Art
Although this invention will, for the sake of clarity of understanding, be described in the context of an olefin production plant (olefin plant), this invention is broadly applicable to the use, as feed thereto, of any hydrocarbon stream containing the requisite feed component(s) as described in detail herein.
Thermal cracking of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butenes, butadiene, and aromatics such as benzene, toluene, and xylene. In an olefin plant, a hydrocarbon feedstock such as naphtha, gas oil, or other fractions of whole crude oil is mixed with steam which serves as a diluent to keep hydrocarbon molecules separated. This mixture, after preheating, is subjected to severe hydrocarbon thermal cracking at elevated temperatures (1450° F. to 1550° F.) in a pyrolysis furnace (steam cracker).
The cracked effluent product from the pyrolysis furnace contains gaseous hydrocarbons of great variety (from 1 to 35 carbon atoms per molecule). This effluent contains hydrocarbons that are aliphatic, aromatic, saturated, and unsaturated, and can contain significant amounts of molecular hydrogen (hydrogen).
The cracked product of a pyrolysis furnace is then further processed in the olefin plant to produce, as products of the plant, various separate individual product streams of high purity such as hydrogen, ethylene, propylene, mixed hydrocarbons having four carbon atoms per molecule (crude C4's), and pyrolysis gasoline. It is this crude C4 product of the debutanizer of an olefin plant upon which is focused this description of one embodiment within this invention.
Crude C4's can contain varying amounts of n-butane (butane), isobutane, butene-1, butene-2, isobutylene (isobutene), acetylenes, butadiene, and hydrogen. The term butene-2 as used herein includes cis-butene-2, trans-butene-2, or a mixture of both.
Heretofore crude C4's have been subjected to butadiene extraction or butadiene selective hydrogenation to remove most, if not essentially all, of the butadiene and acetylenes present. Thereafter the crude C4 raffinate was subjected to an etherification process wherein the isobutylene was converted to methyl tertiary butyl ether (MTBE).
Also heretofore crude C4's have been subjected to selective hydrogenation of dioelfins (butadiene) with simultaneous isomerization of alpha olefins (butene-1) to internal olefins (butene-2) followed by etherification of the isoolefins (isobutylene to MTBE), and finally metathesis of internal olefins (butene-2) with ethylene to produce propylene, see U.S. Pat. No. 5,898,091 to Chodorge et al.
In addition, catalytic distillation of various hydrocarbon streams for various purposes such as hydrogenation, mono-olefin isomerization, etherification, dimerization, hydration, dissociation, and aromatic alkylation has been disclosed, see U.S. Pat. No. 6,495,732 B1.
Finally, olefinic C4's have been converted to propylene and isobutene using selective hydrogenation plus double bond isomerization of butene-1 to butene-2, followed by separation of isobutene from the butene-2, and metathesis of the butene-2 with ethylene to form propylene, see U.S. Pat. No. 6,358,482.
If MTBE market demand should decline, it is desirable to be able to utilize the isobutylene that was formerly used in producing MTBE to produce a different product that is enjoying more robust market demand.
It has been suggested that the isobutylene be dimerized to iso-octene followed by hydrogenation to iso-octane, or be alkylated to iso-octane, neither of which promises to be a cost-effective solution.